The 3rd Generation Partnership Project (3GPP) is continuing development of the fourth-generation wireless network technologies known as Long-Term Evolution (LTE). In heterogeneous networks, a mixture of cells of different sizes and overlapping coverage areas are deployed. Improved support for heterogeneous network operations is part of the ongoing specification of 3GPP LTE Release-10, and improved support and further improvements in the context of new features for heterogeneous network operations are being discussed for Release-11 and beyond.
One example of such a deployment is seen in the heterogeneous network 100 of FIG. 1, where several low-power cells 120 (e.g. picocells), each with a respective coverage area 150, are deployed within the larger coverage area 140 of a macro cell 110, for supporting wireless communication with one or more wireless terminals 160. The macro cell 110 corresponds to a macro-base station, or node B, (“MeNB”), while the picocells 120 correspond to pico base stations (“PeNBs”). The network 100 of FIG. 1 is suggestive of a wide-area wireless network deployment. Some additional examples of low-power nodes, also referred to as “points,” in heterogeneous networks, include home base stations and relays. Although picocells are discussed throughout this application, it is understood that these are only a non-limiting example of the type of low-power cell that can be used in a heterogeneous network.
One aim of deploying low-power nodes such as picocells 120 within the macro coverage area 140 is to improve system capacity, by means of cell-splitting gains. In addition to improving overall system capacity, this approach also allows users to be provided with a wide-area experience of very-high-speed data access, throughout the network. Heterogeneous deployments are particularly effective for covering traffic hotspots, i.e., small geographical areas with high user densities. These areas can be served by picocells, for example, as an alternative deployment to a denser macro network.
Orthogonal Frequency-Division Multiplexing (OFDM) technology is a key underlying component of LTE. As is well known to those skilled in the art, OFDM is a digital multi-carrier modulation scheme employing a large number of closely-spaced orthogonal sub-carriers. Each sub-carrier is separately modulated using conventional modulation techniques and channel coding schemes. In particular, 3GPP has specified Orthogonal Frequency-Division Multiple Access (OFDMA) for the downlink transmissions from the base station to a wireless terminal, and has specified Single Carrier Frequency-Division Multiple Access (SC-FDMA) for uplink transmissions from a wireless terminal to a base station. Both multiple access schemes permit the available sub-carriers to be allocated among several users.
SC-FDMA technology employs specially formed OFDM signals, and is therefore often called “pre-coded OFDM” or Discrete-Fourier-Transform (DFT)-spread OFDM. Although similar in many respects to conventional OFDMA technology, SC-FDMA signals offer a reduced peak-to-average power ratio (PAPR) compared to OFDMA signals, thus allowing transmitter power amplifiers to be operated more efficiently. This in turn facilitates more efficient usage of a wireless terminal's limited battery resources. (SC-FDMA is described more fully in Myung, et al., “Single Carrier FDMA for Uplink Wireless Transmission,” IEEE Vehicular Technology Magazine, vol. 1, no. 3, September 2006, pp. 30-38.)
The basic LTE physical resource can be seen as a time-frequency grid. This concept is illustrated in FIG. 2, which shows a number of so-called “subcarriers” in the frequency domain, at a frequency spacing of Δf, divided into OFDM symbol intervals in the time domain. Each individual element of the resource grid 210 is called a resource element (RE) 220, and corresponds to one subcarrier during one OFDM symbol interval, on a given antenna port. One of the unique aspects of OFDM is that each symbol 230 begins with a cyclic prefix 240, which is essentially a reproduction of the last portion of the symbol 230 affixed to the beginning. This feature minimizes problems from multipath, over a wide range of radio signal environments. Although reference numeral 230 points to a single RE of an OFDM symbol, it is understood that reference numeral 230 refers to the entire OFDM symbol and not just that single RE.
In the time domain, LTE downlink transmissions are organized into radio frames of ten milliseconds each, each radio frame consisting of ten equally-sized subframes of one millisecond duration. This is illustrated in FIG. 3, where an LTE signal 310 includes several frames 320, each of which is divided into ten subframes 330. Not shown in FIG. 3 is that each subframe 330 is further divided into two slots, each of which is 0.5 milliseconds in duration.
LTE link resources are organized into “resource blocks,” often defined as time-frequency blocks with a duration of 0.5 milliseconds, corresponding to one slot, and encompassing a bandwidth of 180 kHz, corresponding to 12 contiguous sub-carriers with a spacing of 15 kHz. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Two time-consecutive resource blocks represent a resource block pair, and correspond to the time interval upon which scheduling operates. Of course, the exact definition of a resource block may vary between LTE and similar systems, and the inventive methods and apparatus described herein are not limited to the numbers used herein. Thus, although the term “resource block” is used below to refer to a block of resources covering 12 consecutive subcarriers with a duration of 1 millisecond (a 12×14 resource block) instead of a duration of 0.5 milliseconds (a 12×7 resource block), it is understood that this is only a non-limiting example.
In general, resource blocks may be dynamically assigned to wireless terminals, and may be assigned independently for the uplink and the downlink. Depending on a wireless terminal's data throughput needs, the system resources allocated to it may be increased by allocating resource blocks across several sub-frames, or across several frequency blocks, or both. Thus, the instantaneous bandwidth allocated to a wireless terminal in a scheduling process may be dynamically adapted to respond to changing conditions.
For scheduling of downlink data, the base station transmits control information in each subframe. This control information identifies the wireless terminals to which data is targeted and the resource blocks, in the current downlink subframe, that are carrying the data for each terminal. The first one, two, three, or four OFDM symbols in each subframe are used to carry this control signaling, and may be referred to as a “control region.” In FIG. 4, a downlink subframe 410 is shown, with three OFDM symbols allocated to control region 420. The control region 420 consists primarily of control data resource elements (REs) 434, but also includes a number of cell-specific reference signal (CRS) REs 432, used by the receiving station to measure channel conditions. These reference signal REs 432 are interspersed at predetermined locations throughout the control region 420 and among the data REs 436 in the data portion 430 of the subframe 410, and correspond to a cell-specific (or common) reference signal (CRS).
Transmissions in LTE are dynamically scheduled in each subframe, where the base station transmits downlink assignments/uplink grants to certain wireless terminals (also known as user equipment, or UEs, in 3GPP terminology) via the physical downlink control channel (PDCCH). The PDCCHs are transmitted in the control region 420 of the OFDM signal, i.e., in the first OFDM symbol(s) of each subframe, and span all or almost all of the entire system bandwidth. A UE that has decoded a downlink assignment, carried by a PDCCH, knows which resource elements in the subframe that contain data aimed for that particular UE. Similarly, upon receiving an uplink grant, the UE knows which time-frequency resources it should transmit upon. In the LTE downlink, data is carried by the physical downlink shared channel (PDSCH) and in the uplink the corresponding channel is referred to as the physical uplink shared channel (PUSCH).
LTE also employs multiple modulation formats, including at least QPSK, 16-QAM, and 64-QAM, as well as advanced coding techniques, so that data throughput may be optimized for any of a variety of signal conditions. Depending on the signal conditions and the desired data rate, a suitable combination of modulation format, coding scheme, and bandwidth is chosen, generally to maximize the system throughput. Power control is also employed to ensure acceptable bit error rates while minimizing interference between cells. In addition, LTE uses a hybrid-ARQ (HARQ) error correction protocol where, after receiving downlink data in a subframe, the terminal attempts to decode it and reports to the base station whether the decoding was successful (ACK) or not (NACK). In the event of an unsuccessful decoding attempt, the base station can retransmit the erroneous data.
LTE offers the possibility of transmitting Multicast and Broadcast over a Single Frequency Network (MBSFN), in which a time-synchronized common waveform is transmitted from multiple cells for a given duration. MBSFN transmission can be highly efficient, allowing for over-the-air combining of multi-cell transmissions in the UE while appearing to the UE as a transmission from a single large cell. Use of MBSFN can improve received signal strength, and can eliminate inter-cell interference.